Lecture 23: Virtual Memory, Multiprocessors Today’s topics: Virtual memory Multiprocessors, cache coherence

Virtual Memory Processes deal with virtual memory – they have the illusion that a very large address space is available to them There is only a limited amount of physical memory that is shared by all processes – a process places part of its virtual memory in this physical memory and the rest is stored on disk (called swap space) Thanks to locality, disk access is likely to be uncommon The hardware ensures that one process cannot access the memory of a different process

Translated to physical Address Translation The virtual and physical memory are broken up into pages 8KB page size Virtual address 13 virtual page number page offset Translated to physical page number Physical address

Memory Hierarchy Properties A virtual memory page can be placed anywhere in physical memory (fully-associative) Replacement is usually LRU (since the miss penalty is huge, we can invest some effort to minimize misses) A page table (indexed by virtual page number) is used for translating virtual to physical page number The page table is itself in memory

TLB Since the number of pages is very high, the page table capacity is too large to fit on chip A translation lookaside buffer (TLB) caches the virtual to physical page number translation for recent accesses A TLB miss requires us to access the page table, which may not even be found in the cache – two expensive memory look-ups to access one word of data! A large page size can increase the coverage of the TLB and reduce the capacity of the page table, but also increases memory waste

TLB and Cache Is the cache indexed with virtual or physical address? To index with a physical address, we will have to first look up the TLB, then the cache  longer access time Multiple virtual addresses can map to the same physical address – must ensure that these different virtual addresses will map to the same location in cache – else, there will be two different copies of the same physical memory word Does the tag array store virtual or physical addresses? Since multiple virtual addresses can map to the same physical address, a virtual tag comparison can flag a miss even if the correct physical memory word is present

Cache and TLB Pipeline Virtually Indexed; Physically Tagged Cache Virtual address Offset Virtual page number Virtual index TLB Tag array Data array Physical page number Physical tag Physical tag comparion Virtually Indexed; Physically Tagged Cache

Bad Events Consider the longest latency possible for a load instruction: TLB miss: must look up page table to find translation for v.page P Calculate the virtual memory address for the page table entry that has the translation for page P – let’s say, this is v.page Q TLB miss for v.page Q: will require navigation of a hierarchical page table (let’s ignore this case for now and assume we have succeeded in finding the physical memory location (R) for page Q) Access memory location R (find this either in L1, L2, or memory) We now have the translation for v.page P – put this into the TLB We now have a TLB hit and know the physical page number – this allows us to do tag comparison and check the L1 cache for a hit If there’s a miss in L1, check L2 – if that misses, check in memory At any point, if the page table entry claims that the page is on disk, flag a page fault – the OS then copies the page from disk to memory and the hardware resumes what it was doing before the page fault … phew!

Multiprocessor Taxonomy SISD: single instruction and single data stream: uniprocessor MISD: no commercial multiprocessor: imagine data going through a pipeline of execution engines SIMD: vector architectures: lower flexibility MIMD: most multiprocessors today: easy to construct with off-the-shelf computers, most flexibility

Memory Organization - I Centralized shared-memory multiprocessor or Symmetric shared-memory multiprocessor (SMP) Multiple processors connected to a single centralized memory – since all processors see the same memory organization  uniform memory access (UMA) Shared-memory because all processors can access the entire memory address space Can centralized memory emerge as a bandwidth bottleneck? – not if you have large caches and employ fewer than a dozen processors

SMPs or Centralized Shared-Memory Processor Processor Processor Processor Caches Caches Caches Caches Main Memory I/O System

Memory Organization - II For higher scalability, memory is distributed among processors  distributed memory multiprocessors If one processor can directly address the memory local to another processor, the address space is shared  distributed shared-memory (DSM) multiprocessor If memories are strictly local, we need messages to communicate data  cluster of computers or multicomputers Non-uniform memory architecture (NUMA) since local memory has lower latency than remote memory

Interconnection network Distributed Memory Multiprocessors Processor & Caches Processor & Caches Processor & Caches Processor & Caches Memory I/O Memory I/O Memory I/O Memory I/O Interconnection network

SMPs Centralized main memory and many caches  many copies of the same data A system is cache coherent if a read returns the most recently written value for that word Time Event Value of X in Cache-A Cache-B Memory 0 - - 1 1 CPU-A reads X 1 - 1 2 CPU-B reads X 1 1 1 3 CPU-A stores 0 in X 0 1 0

Cache Coherence A memory system is coherent if: P writes to X; no other processor writes to X; P reads X and receives the value previously written by P P1 writes to X; no other processor writes to X; sufficient time elapses; P2 reads X and receives value written by P1 Two writes to the same location by two processors are seen in the same order by all processors – write serialization The memory consistency model defines “time elapsed” before the effect of a processor is seen by others

Cache Coherence Protocols Directory-based: A single location (directory) keeps track of the sharing status of a block of memory Snooping: Every cache block is accompanied by the sharing status of that block – all cache controllers monitor the shared bus so they can update the sharing status of the block, if necessary Write-invalidate: a processor gains exclusive access of a block before writing by invalidating all other copies Write-update: when a processor writes, it updates other shared copies of that block

Design Issues Three states for a block: invalid, shared, modified A write is placed on the bus and sharers invalidate themselves Processor Processor Processor Processor Caches Caches Caches Caches Main Memory I/O System

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